Steering device

ABSTRACT

A steering system, in particular a hydraulic rack and pinion steering system for motor vehicles, has the following features:  
     a steering handle (L) for prescribing a steering intention, a pressure medium source ( 18 ) and a control valve device ( 15 ) for controlling a flow of pressure medium to and from a servomotor ( 7 ) in order to deflect vehicle wheels ( 31 ). The control valve device ( 15 ) has a closed center, and a pressure medium accumulator ( 32 ) is provided for producing and/or assisting a flow of pressure medium to and from the servomotor ( 7 ).

[0001] The invention relates to a steering device for a motor vehicle having components of a hydraulic steering system, in particular of a hydraulic rack and pinion steering system.

[0002] In such power assisted steering systems, a pinion is generally mounted rotatably in a steering housing and is held in engagement with a rack which is likewise axially displaceably arranged in the steering housing. A servomotor which is drive-connected to the rack serves to provide power assistance. In this case, a pressure medium source, comprising a pump and a drive, serves, in connection with a control valve device, to control a flow of pressure medium to and from the servomotor as a function of a movement of the pinion.

[0003] EP 0 624 135 B1 has disclosed a power assisted rack and pinion steering system of this type. Here, the control valve device is configured as two control valves which have seat valves, the axes of which are substantially perpendicular to the axis of the pinion. If, by rotating the steering handwheel, the pinion is also rotated, counter to the resistance of the rack, then its floating bearing is displaced. This displacement is transmitted to the control valves. In this way, the control valves are controlled by the steering handle (with the interaction of the rack which offers resistance). As seat valves are used, small movements of the pinion are sufficient to actuate and control the control valves. The movement of the control valves leads to pressure building up in the correspondingly provided cylinder space and thus produces assistive power for deflecting the vehicle wheels by means of the servomotor.

[0004] DE 195 41 749 C1 has disclosed a power steering system for motor vehicles having a servovalve configured as a rotational slide arrangement. It is provided here that a steering handle, arranged without a positive coupling to the steered vehicle wheels, actuates a steering angle setpoint generator and interacts with an actuating motor. The servovalve in this case has what is referred to as an open center, i.e. all connections communicate with one another when the rotational slide and the control sleeve assume a central position relative to one another. An electric motor is arranged in this case at one end of the rotational slide and is capable, on account of corresponding pulses, of rotating the rotational slide and the control sleeve in the corresponding direction.

[0005] The steering system disclosed in DE 195 41 749 C1 is disadvantageously relatively costly, in particular also for interventions in vehicle dynamics. The energy consumption is higher compared with a conventional power steering system. This also results in corresponding heat losses which have to be dissipated in a costly manner, for example by the use of an oil cooler. Steering must be carried out with the aid of the electric motor if the hydraulics or an essential component fails. In this case, the electric motor and the corresponding parts have to be designed for this type of use and correspondingly to be of sufficiently large dimensions. The requirement for high dynamics means that the motors become very expensive. Disadvantageously, the electric motor thus requires a correspondingly large amount of energy from the vehicle-mounted electrical system, with the result that the latter has to be designed to supply sufficient power. The parts relevant to safety, such as the vehicle-mounted electrical system, for example, have to be present in duplicate on account of the safety requirements for such power assisted steering systems, so that higher costs are incurred.

[0006] These disadvantages also hold, at least partially, in analogous fashion for the other power assisted steering systems known from the prior art and for the power assisted steering system disclosed in EP 0 624 135 B1.

[0007] Moreover, it is disadvantageous in the case of the previously known power assisted steering systems that both the instantaneously required energy and the mean energy is relatively high, and in particular the electric motor and the associated parts have to be designed for peak loads (which occur, for example, during parking).

[0008] The invention is based on the object of alleviating the disadvantages of the prior art, in particular of allowing a reliable, simple and cost effective design of a steering system which ensures that the vehicle can be stopped safely even if the hydraulic system fails.

[0009] This object is achieved according to the invention by the features of claims 1 and 12.

[0010] The vehicle can be brought safely to a standstill even if the hydraulic system fails by virtue of the fact that the control valve device has a closed center and a pressure medium accumulator is provided for producing and/or assisting a flow of pressure medium to and from the servomotor. If the hydraulic unit, for example the pump, fails, the energy contained in the pressure accumulator can be used for post-failure operation. It is thus not necessary to design electric motors, which are used for steering if the hydraulic system fails, with correspondingly costly and large dimensions.

[0011] A pressure, which can be used to lower peak loads, for example during parking, can be built up or stored in the pressure accumulator on account of the control valve device being designed with a closed center. The pressure medium source or the pump and the electric motor driving the pump can thus be designed for a mean load. The other participating components, such as the vehicle-mounted electrical system for example, also merely have to be designed for the operation of a pressure medium source with a mean energy requirement.

[0012] It is advantageously unnecessary to use either the vehicle-mounted electrical system or the electric motor for “emergency operation”, i.e. in order to deflect the vehicle wheels if the hydraulic system fails.

[0013] The solution according to the invention thus allows a cost effective, reliable and simple design of the steering system. It is thus in theory possible to dispense with a redundant design of the electric motor and/or of the vehicle-mounted electrical system and the pump, as the energy contained in the pressure accumulator is sufficient to safely bring the vehicle to a standstill. The electric motor advantageously needs to be designed only for actuating the valves and not for supplying steering power.

[0014] It is advantageous that the control valve device is actuated by an actuating motor which is not positively coupled to the steering handle.

[0015] The control valve can be actuated in a particularly simple and cost effective manner by an actuating motor which is not positively connected. The previously existing problem, as resulting, for example, from DE 195 41 749 C1, namely that such actuating motors have to be of powerful and redundant design in order for them to be able to ensure deflection of the vehicle wheels if the hydraulic system fails, is no longer necessary in a particularly advantageous manner as a result of a pressure accumulator and a control valve device with a closed center being used.

[0016] The inventors have discovered in a way that is not obvious that an actuating motor can only be used in series, from a cost point of view, by using a pressure accumulator and a control valve device with a closed center. The actuating motor needs therefore to be designed only for actuating the control valve device and not for deflecting the vehicle wheels. As the actuating motor is normally of redundant configuration, i.e. two are present, considerable costs are thus saved. It is also possible to configure the components which interact with the actuating motor, such as the vehicle-mounted electrical system for example, to be correspondingly smaller and therefore more cost effective. The high dynamic properties of the actuating motor, which were previously necessary as said actuating motor was used to deflect the wheels in emergency operation, are no longer required as a result of the solution according to the invention, and this likewise leads to a considerably more cost effective configuration of the actuating motor or motors.

[0017] It may be provided in a design refinement of the invention for the pressure accumulator to be mounted on the steering mechanism of the steering system.

[0018] Such a refinement has proved to be particularly suitable, reliable and simple to implement in design terms.

[0019] It is advantageous if the pressure accumulator has rigid tubes.

[0020] It has proven more suitable in tests to equip the pressure accumulator with rigid tubes rather than use flexible tubes.

[0021] In particular, the reliability and the handling, for example for installation purposes, are also improved as a result.

[0022] Advantageous developments and refinements of the invention emerge from the further subclaims and from the exemplary embodiments depicted in principle below using the drawing, in which:

[0023]FIG. 1 shows a longitudinal section through a rack and pinion steering system;

[0024]FIG. 2 shows a schematic representation of a steering system with actuation of the control valve device by an actuating motor which is not positively coupled.

[0025] The steering system according to the invention, as described in greater detail in the following exemplary embodiment, has components of a hydraulic rack and pinion steering system for motor vehicles which are in principle of known construction (EP 0 624 135 B1), for which reason details are given below only on the features which are relevant to the invention.

[0026]FIG. 1 shows a steering housing 1 in which a pinion 2 is mounted in a locating bearing 3 and a floating bearing 4. The pinion 2 has an actuating motor 5 at one end. The actuating motor 5 is controlled, via a control circuit R, by means of a steering handwheel L actuated by the driver of the respective vehicle, with the result that said actuating motor 5 rotates in a manner corresponding to the change in steering angle to be set. Reference is made, only by way of example, to DE 195 41 749 C1 concerning the functional principle of such an actuating motor 5.

[0027] The pinion 2 is in engagement, via its toothing, with a rack 6 which is guided in the steering housing 1 in axially displaceable fashion. A servomotor 7, whose piston rod 8 is permanently connected to the rack 6, serves to provide power assistance. The servomotor 7 has two cylinder spaces 9 and 10 which are connected to two cylinder connections 13 and 14 of a control valve device 15 via working lines 11 and 12. The control valve device 15 has two feed line connections 16 and 17, to which a pressure medium source 18 (shown in greater detail, by way of example, in FIG. 2) is connected. The control valve device 15 likewise has two return line connections 19 and 20, to which a container 21 is connected.

[0028] A pressure medium accumulator 32, arranged between the pressure medium source 18 and the control valve device 15, is shown in FIG. 2.

[0029] The control valve device 15 shown in FIG. 1 has two control valves configured as seat valves 22 and 23. The seat valves 22 and 23 each have an inlet seat valve 24 and an outlet seat valve 25. As shown in FIG. 1, the inlet seat valve 24 is held closed in the neutral position by the force of a spring 26. The outlet seat valve 25 is open in the neutral position. This is therefore a control valve device 15 with what is referred to as a closed center.

[0030] A closing body 27 of each outlet seat valve 25 adjoins the floating bearing 4.

[0031] The functioning of the exemplary embodiment shown in FIG. 1 is described in the following text. If the pinion 2 is rotated by a rotational movement of the actuating motor 5, for example in order to steer to the right, then the toothing of the pinion 2 initially rolls along in the toothing of the rack 6 which at that point is still stationary. As a result, as described in greater detail in EP 0 624 135 B1, the seat valves 22 and 23 are acted on in such a way that a pressure is built up in the cylinder space 9, which pressure pushes the rack 6 to the right in accordance with the desired steering movement.

[0032] One advantageous embodiment of the solution according to the invention results from FIG. 2. A steering intention is passed in a known manner to the abovedescribed actuating motor 5. The actuating motor 5 thereupon controls the control valve device 15, which is configured as a rotational slide valve in FIG. 2, in a manner which is preferably influenced by various parameters such as side wind or the like. The rotational slide valve 15 is shown in the neutral position in FIG. 2 and has a closed center. Here, the control edges 28 of the rotational slide valve 15 are moved by a corresponding rotational movement of the actuating motor 5, with the result that the working line 11 or 12 are connected to a feed line 29. A working pressure can thus be built up in a known manner in the corresponding cylinder space 9 or 10 in the servomotor 7. The pressure medium flows from the respective other cylinder space 9 or 10 in a known manner through the respective feed line 11 or 12 into a return line 30 and from there into the container 21. The vehicle wheels 31 indicated in FIG. 2 are deflected by the servomotor 7, under some circumstances in connection with the steering effect of a rack 6.

[0033] A pressure medium source 18 and a pressure medium accumulator 32 are connected to the feed line 29. The pressure medium source 18 and the pressure accumulator 32 produce the pressure necessary to control the servomotor 7 in order to deflect the vehicle wheels 31. Depending on the application, the pressure medium accumulator 32 can assist the pressure medium source 18 or supply the necessary pressure or the necessary energy exclusively. The pressure medium accumulator 32 can thus provide peak loads, when required, in such a way that the pressure medium source 18 merely has to supply a mean output. If the hydraulic system fails, the vehicle wheels 31 can be steered by the pressure contained in the pressure medium accumulator 32, such that it is possible to safely stop the vehicle.

[0034] The pressure medium source 18 has a pump 34, provided with a nonreturn valve 33, and an electric motor 35 supplying the pump 34. A different configuration is naturally also feasible here.

[0035] In the exemplary embodiment illustrated, the pump 34 is configured as a switchable charge pump for a mean energy supply. The peak energy can be obtained here from the pressure medium accumulator 32, which is configured such that the peak load, e.g. for parking maneuvers, is covered without the pressure in the pressure medium accumulator 32 falling to such an extent that it is not sufficient for steering operations.

[0036] In a particularly advantageous manner, the pressure medium accumulator 32 is configured here such that it is possible to steer the vehicle wheels 31 from stop to stop at least three times even if the pressure medium source 18 fails. In tests, this has proved sufficient to bring the vehicle to a standstill without problems.

[0037] In a manner which is not shown in greater detail, it is possible for the pressure medium accumulator 32 to be provided with a device for sensing the accumulator pressure. The pump 34 or the electric motor 35 can thus advantageously be actuated using the accumulator pressure of the pressure medium accumulator 32. It is thus possible to control the pump 34 in a targeted fashion as a function of the accumulator level.

[0038] As the actuating motor 5 does not need to be used to actuate the vehicle wheels 31 or for an emergency steering function, it is possible to design the actuating motor 5 exclusively for actuating the rotational slide valve 15 and hence cost effectively. In tests, it has proved that a torque of at most approximately 5 Nm is sufficient for the actuating motor 5.

[0039] In a manner not shown in greater detail, it may be provided for the actuating motor 5 to be of redundant configuration. This serves to further increase the safety and to fulfill legal requirements.

[0040] It may be provided in alternative embodiments for the pressure medium source 18 to first convey the pressure medium into the pressure medium accumulator 32 and then from there toward the control valve device 15. It is naturally also possible both to connect the pressure medium source 18 to the pressure medium accumulator 32 and also to connect it directly to the control valve device 15 by means of the feed line. As can be seen from FIG. 2, a return line 36 leads from the container 21 to the pump 34 in a known manner.

[0041] Not shown in greater detail in FIG. 2 is the mechanical connection between the control valve device 15 and the rack (likewise not shown) which is coupled to the servomotor 7.

[0042] Without departing from the basic concept of the invention, it is possible to select a different type of mechanism, for example a recirculating ball mechanism, instead of the rack and pinion. In general it is then possible to say driving part instead of “pinion” and driven part instead of “rack”. 

1. A steering device for a motor vehicle having components of a hydraulic rack and pinion steering system and having the following features: A steering handle (L) is provided for prescribing a steering intention, A rotatably mounted pinion (2) is held in engagement with an axially displaceably arranged rack (6), A hydraulic servomotor (7), whose piston rod (8) is drive-connected to the rack (6), serves to provide power assistance, The servomotor (7) has two cylinder spaces (9, 10) which are connected to two cylinder connections of a control valve device (15) via working lines (11, 12), A pressure medium source (18) and the control valve device (15) are provided to control a flow of pressure medium to and from the servomotor (7) in order to deflect vehicle wheels (31), characterized by the following features: The control valve device (15) has a closed center, A pressure medium accumulator (32) is provided for producing and/or assisting the flow of pressure medium to and from the servomotor (7), The pinion (2) has an electric actuating motor (5) at one end, The actuating motor (5) can be controlled, via a control circuit (R), by means of the steering handle (L) which can be adjusted by a driver, The control valve device (15) is actuated by the actuating motor (5) which is not positively coupled to the steering handle (L), The pressure medium accumulator (32) is configured such that the vehicle wheels (31) can be steered from stop to stop at least three times even if the pressure medium source (18) fails.
 2. The steering device as claimed in claim 1, characterized in that the actuating motor (5) is of redundant configuration.
 3. The steering device as claimed in one of the preceding claims, characterized in that the pressure medium source (18) has a pump (34), provided with a nonreturn valve (33), and an electric motor (35) supplying the pump (34).
 4. The steering device as claimed in one of the preceding claims, characterized in that the pump (34) is configured as a switchable charge pump for mean energy supply to the servomotor (7).
 5. The steering device as claimed in one of the preceding claims, characterized in that the pressure accumulator (32) supplied by the pressure medium source (18) is configured such that energy supply to the servomotor (7) for parking maneuvers is reliably ensured.
 6. The steering device as claimed in one of the preceding claims, characterized in that the pressure medium accumulator (32) is provided with a device for sensing the accumulator pressure.
 7. The steering device as claimed in one of the preceding claims, characterized in that the actuating motor (5) has a torque of at most approximately 5 Nm.
 8. The steering device as claimed in one of the preceding claims, characterized in that the pressure medium accumulator (32) is mounted on a steering mechanism.
 9. The steering device as claimed in one of the preceding claims, characterized in that the pressure medium accumulator (32) has rigid tubes.
 10. The steering device as claimed in one of the preceding claims, characterized in that the control valve device (15) is configured as two seat valves (22, 23).
 11. The steering device as claimed in one of the preceding claims, characterized in that the control valve device (15) is configured as a rotational slide valve.
 12. A steering device for a motor vehicle having components of a hydraulic steering system and having the following features: A steering handle (L) is provided for prescribing a steering intention, A rotatably mounted driving part (2) is held in engagement with an axially displaceably arranged driven part (6), A hydraulic servomotor (7), whose piston rod (8) is drive-connected to the driven part (6), serves to provide power assistance, The servomotor (7) has two cylinder spaces (9, 10) which are connected to two cylinder connections of a control valve device (15) via working lines (11, 12), A pressure medium source (18) and the control valve device (15) are provided for controlling a flow of pressure medium to and from the servomotor (7) in order to deflect vehicle wheels (31), characterized by the following features: The control valve device (15) has a closed center, A pressure medium accumulator (32) is provided for producing and/or assisting the flow of pressure medium to and from the servomotor (7), The driving part (2) has an electric actuating motor (5) at one end, The actuating motor (5) can be controlled, via a control circuit (R), by means of the steering handle (L) which can be adjusted by a driver, The control valve device (15) is actuated by the actuating motor (5) which is not positively coupled to the steering handle (L), The pressure medium accumulator (32) is configured such that the vehicle wheels (31) can be steered from stop to stop at least three times even if the pressure medium source (18) fails.
 13. The steering device as claimed in claim 12, characterized in that the driving part and the driven part form a recirculating ball mechanism or a recirculating roller mechanism. 